The invention relates to the art of vehicle braking systems and finds particular application in heavy trucks, tractors, and buses. The invention will be described in relation to an air pressure braking system of a tractor-trailer truck system. It will be appreciated, however, that the invention is also amenable to other applications (e.g., other braking systems such as those of other kinds of vehicles and those based on other technologies such as hydraulic or electronic braking systems).
U.S. Pat. No. 4,844,553 to Bliss and U.S. Pat. No. 4,330,157, which are both incorporated herein by reference, disclose control valve arrangements for use in a braking system. The valve arrangements include a pair of valve actuators or plungers that are operated by a vehicle operator. One of the actuators controls communication between an air pressure source and a tractor parking/emergency brake system. The tractor parking/emergency brake system includes spring brakes that are actuated to resist wheel rotation. A force of an included spring or set of springs in the spring brakes urges a braking member against the wheel. During vehicle operation, air pressure is applied against a piston that is associated with the springs. The force applied against the piston counteracts the force of the springs thereby releasing the spring brake and allowing the wheels of the vehicle to rotate. The other actuator controls communication between the pressure source and service brakes of a trailer.
In addition to providing for manual application and release of the parking/emergency brakes, the disclosed valve arrangements also provide for the automatic application of brakes should the source or actuation pressure fall below a threshold. That is to say, the first actuator is forced into a pressure exhaust position by internal springs if the source pressure drops below a first threshold. Similarly the other actuator is forced into a pressure exhaust position if the pressure drops below a second threshold. This provides for the trailer brakes and the tractor brakes to be applied in an appropriate order should a malfunction or leak occur in the braking system.
One feature the prior art valve arrangements do not provide is an assurance that a vehicle operator has actuated the service brakes before the parking brakes are released. As a result, the vehicle could roll unexpectedly when the operator releases the parking brakes.
Therefore, a braking system interlock arrangement that requires the service brakes of a vehicle to be engaged before the parking brakes of the vehicle can be disengaged is desired.
The present invention provides a new and improved apparatus and method which addresses the above-referenced problems.